This invention relates to electrochemical cells, such as proton exchange membrane (PEM) fuel cells, direct methanol fuel cells (DMFC) and, more particularly, to electrochemical cells operating at ambient temperatures and pressures and which comprise membrane-electrode assemblies whose electrodes are lightly loaded with catalyst.
The performance of a proton exchange membrane (PEM) fuel cell and a direct methanol fuel cell (DMFC) is largely determined by the membrane-electrode assembly (MEA), which is composed of an anode that oxidizes the fuel, a cathode that reduces oxygen, and a proton transferring, ion-conducting membrane. The membrane prevents electrical short-circuiting between the anode and the cathode, and separates the fuel from the oxidant.
A PEM fuel cell normally uses hydrogen as the fuel, while a DMFC uses methanol as the fuel. The terms xe2x80x9cfuel cellxe2x80x9d, or xe2x80x9ccellxe2x80x9d, as used hereinafter, shall define either a PEM or DMFC-type fuel cell.
The fuel oxidation reaction and the oxygen reduction reaction of these cells, typically, are kinetically slow. Therefore, catalysts such as platinum and its alloys are often used to speed up these reactions.
In the fuel cells, the catalysts are generally made into porous layers that serve to increase the contact area between the reactants and the catalyst particles. The layers can be applied directly to the membrane or they can be applied to a gas diffusion medium. Carbon paper and carbon cloth-type materials are commonly used as gas diffusion media because of their good electrical conductivity, high corrosion resistance, and controllable porosity.
It is desirable to decrease the amount of noble metals used to fabricate the fuel cells in order to reduce the cost of manufacture. Catalyst layers comprising metal black with lower surface areas were initially used in the fuel cells. The catalyst loading for an electrode was required to be over 4.0 mg/cm2 in order to achieve good performance.
Subsequently, catalyst layers containing supported metal nano-particles featuring higher surface areas were prepared, as illustrated in U.S. Pat. No. 4,166,143, granted to Petrow et al on Aug. 28, 1979, U.S. Pat. No. 4,876,115, granted to Raistrick on Oct. 24, 1989, and U.S. Pat. No. Re. 33,149 granted to Petrow et al on Jan. 16, 1990. The support has several functions. It provides sites for anchoring the metal particles during and after their formation. The particles formed in such an environment have small and uniform size. Particle coalescence, or aggregation, becomes less likely. Therefore, the catalyst does not lose its surface area as fast as an unsupported catalyst due to the chemical/physical interaction between the support and the metal particles. The support also provides electrical connection to catalyst particles carried upon different support materials. Carbon black is the most practical support material used in fuel cells because of its large surface area, good electrical conductivity, and high corrosion resistance.
Electrons and protons are both involved in fuel cell reactions, so it is necessary to provide good electrical and proton conductivities. This limits the reaction zone within the interface between the catalyst layer and the ion-conducting membrane for a traditional MEA. This interfacial region is extremely thin and the total surface area of the catalyst particles in this region is low. Thus, the catalyst layer cannot provide a high current density. The catalyst that is not in contact with the membrane is simply wasted. This can be changed by incorporating an ionic conductor, such as Nafion(copyright), a perfluoronated ionomer made by E. I. DuPont, into the catalyst layers.
After Nafion incorporation, the entire catalyst layer conducts both electrons and protons so that catalyst utilization in the layer is improved dramatically. The catalyst layer will in turn generate and sustain a higher current density. Nafion can be impregnated into a catalyst layer by brushing and spraying it upon an electrode surface or by respectively floating or dipping the electrode upon or into a Nafion solution. Some illustrations were made by Ticianelli et al., Methods to advance technology of proton exchange membrane fuel cells, J. Electrochem., Soc. pp. 2209-2214 (1988), September; and Poltarzewski et al., Nafion(copyright) distribution in gas diffusion electrodes for solid polymer-electrolyte-fuel-cell applications, J. Electrochem. Soc. pp 761-765 (1992), March.
The advantage of applying Nafion this way provides an opportunity to incorporate polytetrafluoroethylene (PTFE), which is a water repelling agent, into the catalyst layer. This is usually accomplished before the application of Nafion so that the final catalyst layer will have a controllable hydrophobic character in order to reduce the likelihood of flooding. The disadvantage of this method, however, is that it is very difficult to control the amount of Nafion that is applied. This results in a non-homogeneous distribution of the Nafion material over the entire catalyst layer. The regions containing more Nafion material will be easily flooded, while the regions with insufficient Nafion material may not be able to provide enough proton conductivity. The Nafion may localize on the surface in one place, but penetrate the underlying gas diffusion medium in another place.
Another method of incorporating Nafion into the catalyst layer is to mix the catalysts directly with the Nafion, especially supported catalysts, and then use the resulting mixture to fabricate the catalyst layer, as illustrated in U.S. Pat. No. 5,211,984, granted to Wilson on May 18, 1993, U.S. Pat. No. 5,723,173, granted to Fukuoka et al on Mar. 3, 1998, U.S. Pat. No. 5,728,485, granted to Watanabe et al on Mar. 17, 1998, and U.S. Pat. No. 6,309,772, granted to Zuber et al on Oct. 30, 2001. The catalysts and Nafion mixture is easily achieved; they can form an even distribution through the entire catalyst layer. Solvents such as glycerol may be used during the mixing in order to achieve good viscosity and hold the catalyst particles in suspension in order to minimize their agglomeration, as shown in U.S. Pat. No. 5,211,984, granted to Wilson on May 18, 1993.
Sometimes, the Nafion solution is converted into a colloidal suspension by adding a proper organic solvent before mixing it with catalysts, as illustrated in U.S. Pat. No. 5,723,173, granted to Fukuoka et al on Mar. 3, 1998. This patent teaches that such a colloidal suspension can provide a good Nafion network for achieving a uniform distribution with the catalyst particles.
Directly mixing the Nafion with the catalyst, however, makes it almost impossible to incorporate PTFE into the catalyst layer. This is because PTFE needs to be sintered at a temperature in excess of 330xc2x0 C., but such an elevated temperature will destroy Nafion. Without the PTFE, the catalyst layer is more likely to be flooded.
All of these recent developments have helped to decrease the catalyst loading from 4.0 mg/cm2 or higher to 0.5 mg/cm2 or less. However, a fuel cell that has slightly loaded catalyst electrodes and operates under ambient conditions shows much lower performance than a fuel cell using highly loaded catalyst electrodes. Even though the catalyst layer is made to conduct both protons and electrons through the mixing of the supported catalyst with Nafion, many of the catalyst sites will not become active under ambient conditions.
The present invention provides a procedure and an article made by the procedure, whereby catalyst utilization can be increased. The method comprises the activation of the membrane-electrode assembly. The activation procedure is much shorter than traditional xe2x80x9cbreak-inxe2x80x9d processes, yet also dramatically increases catalyst utilization.
Mixing a proton conducting material, such as Nafion, with a catalyst layer will improve fuel cell performance. However, providing a three-dimensional catalyst layer to conduct protons will not necessarily provide a well performing electrode. Many of the catalyst sites in such a layer will not be available for reaction. Some of a number of reasons could be that: a) the reactant cannot reach the catalyst sites because they are blocked; b) the Nafion near these catalyst sites cannot be easily hydrated; or c) the ionic or electronic continuity is not established at these sites.
Catalyst sites that cannot participate in the electrochemical reaction of the fuel cell are xe2x80x9cdeadxe2x80x9d. Making these xe2x80x9cdeadxe2x80x9d sites active is one of the objectives of this invention. It has been found that temperature and pressurization play a crucial role in converting xe2x80x9cdeadxe2x80x9d sites into active sites. Operating a fuel cell above ambient temperature, preferably close to 100xc2x0 C., and applying a few atmospheres of pressure to the gaseous reactants, will quickly provide an activated fuel cell after only a number of hours. Such an activation procedure results in a dramatic increase in catalyst utilization. The fuel cell can then generate a much higher current density under ambient conditions.
One objective of the present invention is to provide increased catalyst utilization by a new activation procedure, especially for the electrodes of fuel cells with low catalyst loadings and fabricated with supported catalysts.
Another objective of this invention is to provide a means to boost the MEA of a fuel cell to a high performance within a few hours.
Still another objective of the present invention is to provide a fuel cell starting procedure, especially one where the fuel cell is intended to be operated under ambient conditions (e.g., ambient temperature and pressure).